Invisible With Household Ingredients: Mechanisms and Properties

Ben Artin Northeastern University College of Professional Studies CMH1011: Chemical Principles 1 Instructor: Patricia Brandl June 24, 2011

Abstract

Invisible inks — colorless substances usable for writing that can be turned to color using a chemical process — have both practical value in covert communications and educational value in chemistry teaching. Publicly available invisible recipes are numerous and varied, but are often published without any elaboration as to their utility and usability. This paper evaluates approximately a dozen different invisible ink recipes, with focus on those that can be prepared from common household ingredients. Safety, cost, availability, shelf-life, and dif- ficulty of use of the different invisible inks are considered. Based on this evaluation, a general recommendation is made in favor of heat-based inks for casual covert communications and chemistry education, and precipitation-based inks for more secure covert communications and chemistry education in a lab setting. Additionally, two proposed mechanisms for action of invisible inks based on organic and revealed by heating are evaluated. Oxidation of ink by heat is confirmed to play a role in revealing such inks.

1 Introduction ing mathematics2, computer science3, quan- tum physics4, and chemistry5. Parents want to know about the activities of In the realm of chemistry, a popular tech- their children; generals want to know about the nique for covert communication — seen in sci- activities of their troops. Throughout human ence education6, intelligence operations5, and history, on scale from personal to international, mass-market fiction7 — are invisible inks. knowledge has been a key ingredient in power. An invisible ink is any substance that can In struggle to establish and maintain power, be used for writing (typically on paper) and people often desire to communicate covertly is not easily detected by the naked eye. The and intercept others’ covert communications. process of rendering the ink visible (ideally by As a result, numerous technological advances the intended recipient) is known as developing have been developed primarily to deal with the ink; the apparatus or substance used to de- covert communication — both to enable it, velop the ink is the developer. and to intercept it. Recorded accounts of such This paper reviews several substances that technology go as far back as 1500 BCE1, and can be used as invisible inks, as well as touch nearly every scientific discipline, includ- their corresponding developers, with focus on

1 – those that can be manufactured from common species, Ind1 and Ind2 , of which one can be household products and other readily available obtained by protonation of the other: reagents. − + Ind1 Ind2 + H 1.1 Classification of inks by mechanism of action and development and those two species have different colors be- cause their different bond and electronic struc- Many ink/developer pairs share the same tures result in their having different absorption underlying chemical mechanism of action (on spectra in visible frequencies of light. writing surface) and development. Such re- When Ind1 is exposed to an acidic or basic lated pairs are often similar to each other on environment, the equilibrium concentrations of several of the evaluation criteria. It is, there- – Ind1 and Ind2 depend on the pH of the envi- fore, convenient to group their evaluation by ronment, and therefore so does the color of the mechanism. – 15 mixture of Ind1 and Ind2 . Mechanisms present among the inks and de- Substances containing acids or bases (such velopers evaluated in this paper include: as citric or ) can be used as in- visible inks and developed by exposing them to Sugar solutions developed by heat a suitable pH indicator. To develop a pH-based invisible ink, a pH Substances containing sugars (such as honey indicator in aqueous solution is applied both solution and sugar solution) can be developed to the marked regions of the writing surface by heating. When applied to paper and then and the unmarked ones. The indicator has to heated, these solutions turn brown8. be chosen so that the ink produces a different The reaction responsible for this change of indicator color than the unmarked writing sur- color is presumably caramelization (a non- face does, which depends both on pH of the ink enzymic reaction that results in browning of and the pH of the writing surface. sugars9).

Organic acids developed by heat Inorganic salts developed by a precipitation reaction Substances containing organic acids (such as citric acid in lemon juice) can be developed Water-soluble salts can be developed using by heating. When applied to paper and then a precipitation reaction, if the salt solution is heated, these inks turn brown10. colorless and a colored precipitate can be pro- The mechanism of action and development duced. is unclear; two different possibilities are com- The ink, containing an ion (IonI ) in aque- monly cited — that the organic acid itself ous solution is applied to a writing surface and browns with heating11,12, and that the acid re- allowed to dry. To develop the ink, aqueous acts with the polymers of the paper, converting solution of a different ion (IonD) is applied to them into compounds that brown on exposure the writing surface. If the two ions combine to heat13,14. into a salt (SaltP ) that has low solubility in water and forms a colored precipitate: Acids and bases developed by a pH Ion + Ion Salt ↓ indicator I D P

pH indicators are substances whose color then the appearance of colored precipitate of changes on exposure to acidic or basic solu- SaltP on the writing surface makes the writ- tions. Specifically, an indicator exists as two ing visible.

2 2 Methods ˆ R 22: Harmful if swallowed.

2.1 Ink Evaluation ˆ R 23/25: Toxic by inhalation and if swal- lowed. Inks and developers were assessed on the fol- lowing criteria: ˆ R 25: Toxic if swallowed.

Availability ˆ R 36/37: Irritating to eyes and respira- tory system. Availability of a reagent is determined to be “common” if it is readily available from com- ˆ R 36/38: Irritating to eyes and skin. mon brick-and-mortar and internet stores, such as grocery stores; and “specialty” if it is only ˆ R 36/37/38: Irritating to eyes, respira- available from specialty stores, such as garden- tory system and skin. ing stores. ˆ R 37/38: Irritating to respiratory system Cost and skin.

Except where otherwise specified, cost is ˆ R 41: Risk of serious damage to eyes. based on retail prices during the week of Jan 20–26 2001 at Walgreens and Stop and Shop in ˆ R 46: May cause heritable genetic dam- Malden, MA. age.

Preparation ˆ R 62: Possible risk of impaired fertility. Preparation is classified as “simple” if it involves nothing beyond mixing with water Contrast at room temperature, “medium” if common Contrast of the writing was classified as household processes, such as boiling water, are “high” if an ink yielded strokes with high required, and “difficult” if specialized equip- contrast against the background, “low” if the ment (other than safety equipment) is required. strokes had low contrast against the back- ground, and “invisible” if strokes were com- Shelf-life pletely invisible. Shelf-life was estimated based on prior ex- perience with household products in question, Stroke width assuming that the ink is not kept refrigerated. Stroke width of the writing was classified as Safety “broad” if only broad strokes (applied with a cotton swab) were discernible, and “thin” if Risks of inks and developers were deter- thin strokes (applied with a nib ) were dis- mined and summarized by the following stan- cernible. dard16,17 risk phrases:

ˆ R 8: Contact with combustible material Sharpness may cause fire. Sharpness of the writing was classified as ˆ R 20: Harmful by inhalation. “sharp” if strokes of ink with food dye were not visibly distorted or blurred by developing, ˆ R 20/22: Harmful by inhalation and if and “blurred” if they were blurred by develop- swallowed. ing.

3 2.2 Choice of inks and developers & Shop— 100% Pure Corn , UPC 688267070365) In order to be useful for development of pH- based inks, pH indicators have to be available ˆ Sugar, 10 g dissolved in 100 ml H2O in aqueous solution. Common aqueous labo- (Domino® Premium Granulated Pure ratory indicators (such as and Cane Sugar, UPC 049200045701) methyl red) are deemed unsafe for consumer 18 ˆ products in various world regions , and there- Honey, diluted to 20 % by H2O (Gunter’s fore are typically only available in specialized Pure Honey, UPC 021273100129) stores, such as online lab supply stores. They ˆ are toxic, often carcinogenic, and irritants, as White onion water (filtrate of 50 g well as being relatively expensive. Such devel- chopped jumbo white onion boiled in opers were not evaluated. 500 mL H2O) A viable alternative to these is preparation ˆ Lemon juice, diluted to 20 % by H O (Si- of aqueous solution of flavin, an anthocyanin 2 cilia Lemon Juice, UPC 030849000053) present in red cabbage19. Red cabbage is commonly and cheaply available in brick-and- ˆ ® Milk, diluted to 20 % by H2O (Hood mortar stores. An extract of flavin from red Fat Free Milk, UPC 044100169250) cabbage can be prepared with a simple stove- top process. Its shelf life is unknown, and it is along with the following developers: not hazardous beyond risks involved with boil- ˆ ing water on a stove. pH indicator: Red cabbage water (fil- Numerous ionic compounds exist that par- trate of 170 g chopped red cabbage boiled ® ticipate in precipitation reactions and there- in 500 mL H2O; Stop & Shop Fresh fore could be used as invisible inks, but many Red Cabbage, UPC 02112041383) of them are not readily available in consumer ˆ : Iodine tincture diluted to 5 % products due to their toxicity (such as cop- by H O (Swan Iodine Tincture, UPC per(II) sulfate and potassium ferricyanide); 2 30869385110) they can typically only be obtained from spe- cialty online stores. Similarly, numerous ionic ˆ Heat: 20 min in a kitchen oven heated to compounds exist that can be used as invisible 200 ‰ ink developers, but are also often hazardous, expensive, and difficult to obtain. Each ink was applied to a piece of acid-free Such inks and developers (only available paper of brightness 92 and weight 20 lb four from laboratory supply stores) were not evalu- times: ated. ˆ The following invisible inks were used: Using a cotton swab dipped in ink

ˆ White vinegar (Stop & Shop— Distilled ˆ Using a nib pen dipped in ink White Vinegar, UPC 688267045745) ˆ Using a cotton swab dipped in ink with ˆ Clear Ammonia, diluted to 5 % by food coloring added H O (Walgreens Clear Ammonia, UPC 2 ˆ Using a nib pen dipped in ink with food 049022352773) coloring added ˆ Baking soda, 10 g dissolved in 300 ml — Developing a dyed ink allowed for the shape H2O (Guaranteed Value Baking Soda, UPC 688267067570) of the writing to be compared before and af- ter development, to ascertain how well a given ˆ Starch, 10 g dissolved in 50 ml H2O (Stop developer preserves strokes. Comparison of

4 cotton swab application and nib pen applica- 3.3 Evaluation of inks developed by tion allows determination of how well different precipitation strokes are preserved. Food coloring used was McCormick Red All precipitation-based invisible inks are dif- Food Color (UPC unknown). ficult to obtain; none were directly evaluated. See Table 2 for partial evaluation of precipitation-based inks available in specialty 2.3 Investigation of heat-based stores. development of inks based on organic acids 3.4 Evaluation of other inks developed To investigate two proposed mechanisms for by liquid developer heat-based development of inks based on or- ganic acids, aqueous inks in a container One other ink developed by liquid developer were treated by heat under conditions identi- was evaluated; see Table 3. cal to those used to develop them on paper. Because one of the proposed mechanisms re- 3.5 Evaluation of inks developed by quires a with paper, if it were heat the only mechanism of heat-based development of these inks, inks heated in absence of paper Reagents for sugar-based inks (sugar or would not undergo the same color change. honey) are readily and cheaply available in common brick-and-mortar stores. Inks are 3 Results quick and easy to prepare, but will spoil within 1-2 weeks, as the sugars are a growth medium 3.1 Mechanism of heat-based for bacteria, fungi, molds, and yeast. Inks and development of inks based on organic ink reagents are safe in quantities likely to be acids encountered in household production of invisi- ble ink, except for risks associated with inhala- Inks based on milk and lemon juice, when tion of powdered sugar, and risks of inhalation applied to paper and heated in an oven at and ingestion of spoiled ink. 200 ‰, browned in 10–20 min. Reagents for pH-based inks (such as vinegar When the same inks were heated under the and lemon juic) are readily and cheaply avail- same conditions in a glass dish, they browned able in common brick-and-mortar stores. Inks in a similar time period. are quick and easy to prepare, and generally keep better than sugar-based inks. Ink and 3.2 Evaluation of inks developed by pH reagents are often irritants. indicator Heat-based development of invisible inks can Reagents for pH-based inks (such as vine- be performed using an incandescent or halo- gar and ammonia) are typically readily and gen bulb, an oven, or an iron. All of these can cheaply available in common brick-and-mortar cause burns if handled improperly. All of them stores. Inks are quick and easy to prepare, and also run the risk of igniting the paper in the do not spoil due to their pH. Inks and reagents, process of ink development, which is likely to by virtue of being acidic or basic, are often cause bodily harm, as well as destroy the secret toxic and irritants. message. See Table 1 for detailed evaluation of - Detailed evaluation of heat-developed inks is based inks. listed in Table 4.

5 Reagent white ammonia baking soda vinegar

Active ingredient CH3COOH NH4OH NaHCO3 Ink Availability common common common Cost 0.002 USD/ml 0.001 USD/ml 0.002 USD/g Preparation simple simple simple Shelf-life indefinite indefinite indefinite Safety R 20/22, R 36 R 36/37/38 R 36/37/38 Reagent red cabbage extract Active ingredient flavin Availability common Developer Cost 0.004 USD/mL Preparation medium Shelf-life unknown Safety unknown Contrast Low Invisible Low Legibility Stroke width Broad — Broad Sharpness Blurred — Blurred

Table 1: Evaluation of inks developed by pH indicator

Reagent algicidesa copperasb

Active ingredient CuSO4 FeSO4 Availability specialty specialty Ink Cost 0.013 USD/g 0.002 USD/g Shelf-life indefinite indefinite Safety R 25, R 36/37 R 36/38, R 23/25 Reagent washing sodac

Active ingredient Na2CO3 Precipitate formed CuCO3 FeCO3 Developer Availability common Cost 0.002 USD/g Preparation simple Shelf-life indefinite Safety R 20/22, R 37/38, R 41 a Such as Crystal Blue Copper Sulfate Smart Crystals, UPC unknown; cost estimate based on June 24 2011 price on amazon.com b Such as Hi Yield Copperas Iron Sulfate, UPC unknown; cost estimate based on June 24 2011 price on marshallgrain.com c Such as Arm & Hammer® All Natural Super Washing Soda, UPC 033200030201

Table 2: Evaluation of inks developed by precipitation

6 Reagent starch Active ingredient starch Availability common Ink Cost 0.002 USD/g Shelf-life indefinite Safety R 36/37 Reagent iodine tincture – – Active ingredient I ,I3 Availability common Developer Cost 0.13 USD/ml Preparation simple Shelf-life indefinite Safety R 22, R 36/38, R 46, R 62 Contrast Invisible Legibility Stroke width — Sharpness —

Table 3: Evaluation of inks developed by other liquid developer

7 Reagent sugar honey lemon juice onion juice milk Active ingredient sugars sugars unknown unknown unknown Availability common common common common common Ink Cost 0.012 USD/g 0.012 USD/g 0.008 USD/ml 0.002 USD/ml 0.002 USD/ml Preparation simple simple simple medium simple Shelf-life weeks weeks months months days

8 Safety R 20 R 20 R 36/37 R 20 Developer Safety R 8 R 8 R 8 R 8 R 8 Contrast High High Low Low Low Legibility Stroke width Thin Thin Thin Thin Thin Sharpness Sharp Sharp Sharp Sharp Sharp

Table 4: Evaluation of inks developed by heat 4 Conclusions life was not evaluated. However, revealing the writing requires concentrations of the ink and 4.1 Mechanism of development of inks the developer to be matched, so as to produce a based on organic acids change in indicator color. (For example, when Inks based on organic acids, when treated by ammonia ink and baking soda ink were devel- heat, brown in absence of paper; therefore, a oped using a particular flavin solution, only possible reaction of the acids with the paper is baking soda yielded visible strokes.) Further- not the only source of browning (although such more, because the developer in aqueous solu- a reaction may be present); inks’ oxidation by tion has to be applied to the paper, strokes of heat contributes to the change of color. invisible ink are blurred. As a result, only thick strokes of invisible inks were typically legible when developed by flavin. 4.2 Ink recommendations

Readily available inks and developers were Overall, this makes pH-based ink inconve- found to be so cheap that cost is unlikely to be nient to use, and unsuitable for applications a concern for any except iodine. that require fine strokes. They do, how- Overall, precipitation-based invisible inks ever, provide a practical demonstration of well- are the most difficult to obtain of those evalu- understood chemistry (acids, bases, and pH), ated. Given that other ink/developer pairs ex- and are therefore useful in education. ist that are safe, legible, and readily accessible, precipitation-based invisible inks are unlikely to hold much practical value in most applica- Heat-based development is readily available tions. However, as some of these inks can only in many cases, and although potentially haz- be developed by a small number of uncommon ardous, the hazards can be mitigated by us- developers, they might be preferable in situ- ing a uniform and controlled heat source (such ations in which it is important for the covert as a conventional oven, as opposed to a can- communications to be resistant to attempts to dle light or an incandescent bulb). The inks reveal them by trying several common devel- it can be used on are safe and common, but opers. many have limited shelf-life. The development Another value of precipitation-based inks is process preserves thin strokes very well. Thin in chemistry education, where they can be used strokes make detection of covert communica- to associate basic concepts of general chemistry tions harder, because they distort the writing (substitution reactions and precipitation) with surface much less than broad strokes; therefore, a practical application (although one of limited heat-based inks are a good choice for covert use). writing, but would not be suitable where an Inks developed by iodine are safe and readily adversary is expecting covert communications available, but iodine stains skin, is toxic, and and therefore likely to try common developers is less readily available than the inks. Over- such as heat. all, these inks are less convenient to use than pH-based or heat-based inks. Mechanisms involved in heat-based develop- Educationally, iodine-developed inks can be ment of inks are either poorly understood or interesting as a way of demonstrating impor- chemically complex, and therefore provide a tant chemical concepts, such as starch-iodine simple demonstration of chemistry that can- reaction used in redox titration of iodine. not be explained simply. A heat-based invisi- pH-based invisible inks are readily available, ble ink demonstration might be useful for get- have a long shelf-life, and are often safe. The ting someone interested in science, but curios- only readily available aqueous pH indicator ity about the details of the underlying chem- (flavin) is safe and easy to prepare; its shelf- istry is likely to go unsatisfied.

9 5 Further research None of the precipitation-based inks were readily available, but it is possible that some While red cabbage extract is a popular of them can be manufactured from common in- household source of flavin, there are others gredients using other chemical processes. This (such as grapes). To make an appropriate rec- was not considered for this paper, but if true ommendation among the different sources of would make it possible to create more secure flavin, the extracts should be prepared and inks from household ingredients. their properties evaluated.

References

[1] David Kahn. The Codebreakers. Scribner, 1996.

[2] Heffrey Hoffstein, Jill Pipher, and J.H. Silverman. An Introduction to Mathematical Cryp- tography. Springer Verlag, 2008.

[3] Arto Salomaa. Public-Key Cryptography. Springer Verlag, 1996.

[4] Anton Zeilinger, Artur K. Ekert, and Dirk Bouwmeester. The Physics of Quantum Informa- tion: Quantum Cryptography, Quantum Teleportation, Quantum Computation. Springer Verlag, 2000.

[5] June B. Bell. Invisible-ink Formula Stays Invisible. National Law Journal, 24(26), March 2002.

[6] Trevor Cook. Experiments with States of Matter. PowerKids Press, 2009.

[7] Joanne K. Rowling. Prisoner of Azkaban. Bloomsbury Publishing PLC, 1999.

[8] Robert Gardner and Barbara Gardner Conklin. Chemistry Science Fair Projects: Using French Fries, Gumdrops, , and Other Organic Stuff. Enslow Publishers, 2004.

[9] M. del Pilar Buera, Jorge Chirife, Silvia L. Resnik, and Roberto D. Lozano. Nonenzymatic browning in liquid model systems of high water activity: Kinetics of color changes due to caramelization of various single sugars. Journal of Food Science, 52(4):1059–1062, 1987.

[10] Cy Tymony. Sneakier Uses for Everyday Things. Andrews McMeel Publishing, 2005.

[11] Anne Marie Helmenstine. Lemon Juice Invisible Ink, . URL http://www.topspysecrets. com/lemon-juice-invisible-ink.html.

[12] Create Invisible Ink With Lemon Juice — Fun Science Experiments for Kids, . URL http://www.sciencekids.co.nz/experiments/invisibleink.html.

[13] Invisible Ink — Wikipedia, the Free Encyclopedia, . URL http://en.wikipedia.org/ wiki/Invisible_ink.

[14] Anne Marie Helmenstine. Make Invisible Ink — Lemon Juice, . URL http://chemistry. about.com/cs/howtos/ht/invisibleink3.htm.

[15] Daniel C. Harris. Exploring Chemical Analysis. W H Freeman Co, 2004.

10 [16] Commission Directive 2001/59/EC of 6 August 2001: Adapting to Technical Progress for the 28th time Council Directive 67/548/EEC on the Approximation of the Laws, Regula- tions and Administrative Provisions Relating to the Classification, Packaging and Labelling of Dangerous Substances, . URL http://eur-lex.europa.eu/LexUriServ/LexUriServ. do?uri=CELEX:32001L0059:EN:HTML.

[17] List of R-phrases — Wikipedia, the Free Encyclopedia, . URL http://en.wikipedia. org/wiki/List_of_R-phrases.

[18] FDA Proposes Ban on OTC Sale of Laxative Ingredient, 1997. URL http://archive. hhs.gov/news/press/1997pres/970829.html.

[19] New scientist. 191(2558-2566):65, 2006.

11